Nanoparticles of a metal compound, e.g., a metal oxide, a doped metal compound, and a metal complex, are widely used in the fields of chemical catalysts, optoelectronic materials, optical materials, sensor materials, flame retardant materials, electrode materials and others. Such nanoparticles are provided in various shapes which include, e.g., spherical particles, nanofibers, and nanosheets having enhanced surface activity.
A method of preparing such nanoparticles comprises placing a metal salt solution in an autoclave, heating to a pre-determined reaction temperature, and performing hydrothermal synthesis or solvothermal synthesis. However, this method produces coarse metal compound particles having a large particle size and a low specific surface area.
There have recently been developed various methods of preparing such nanoparticles, examples of which include a sol-gel method (US Patent Publication No. 2007/0092423A1), a co-precipitation method (US Patent Publication No. 2006/0150526A1), a hydrothermal synthesis method (US Patent Publication No. 2005/0003744A1), and a spray pyrolysis method (Korean Unexamined Patent Publication No. 2004-0069404).
US Patent Publication No. 2007/0092423A1 discloses a method of preparing spherical or fibrous oxide nanoparticles by conducting a hydrolytic or non-hydrolytic sol-gel reaction, which comprises heating a mixture of a metal precursor and a surfactant in an organic solvent to produce a metal-surfactant complex, which is then combined with an ester and heated, to obtain nanoparticles. However, this method is problematic in that the use of expensive materials is required, the process is complicated, and the formation of the metal-surfactant complex is sluggish.
US Patent Publication No. 2006/0150526A1 discloses a method of producing oxide particles via precipitating a metal ammonium carbonate salt, which is then heat-treated. This patent discloses the production of particles having a very high specific surface area. However, the particles thus formed are of the form of a large aggregate, and it is difficult to obtain particles having good dispersibility and a uniform particle size.
US Patent Publication No. 2005/0003744A1 discloses a method of preparing composite particles by treating a cerium-titanium-mixed precursor with potassium hydroxide, and subjecting the reaction mixture to hydrothermal synthesis at a high temperature of about 300° C. over a long period of time. Although this method is useful for doping a titanium oxide with cerium oxide, it is disadvantageous because the synthesis requiring a long period of reaction under a high temperature and high pressure condition, leading to increased energy cost and generation of impurities.
Korean Unexamined Patent Publication No. 2004-0069404 discloses a spray pyrolysis method, which comprises spraying a metal precursor through a nozzle, to obtain spherical granules which are subsequently heat-treated, to obtain an oxide. This method is advantageous because the process is simple and chemically uniform oxide particles may be obtained, but suffers from the problem that the particle size is non-uniform and a large aggregate is formed.
In addition, there is reported a supercritical hydrothermal reaction method for preparing metal oxide particles by treating an aqueous metal salt solution with aqueous alkali under a supercritical condition of water. This method may be performed using a continuous process, but the synthesis performed under a high-temperature and high-pressure condition of 380° C. and 500 bar or more, requires the use of expensive equipments and a high process energy cost.
As mentioned above, the conventional techniques each has problems, and thus, there is a need for an improved, economical method of preparing metal compound nanoparticles having a desired shape.